In recent years, apparatuses to which wireless technology is applied have rapidly come into widespread use. Such wireless technology includes a wireless LAN system complying with the IEEE802.11a/b/g standards, Bluetooth and so on. According to the IEEE802.11a or the IEEE802.11g, a data transmission rate is defined as 54 Mbps, however, research and development for realizing the higher transmission rate have been recently energetically pushed forward.
As one of techniques for realizing speeding-up of a wireless communication system, a MIMO (Multi-Input Multi-Output) communication system attracts increasing attention. According to this technique, improvement in communication rate is achieved by improving transmission capacity by realizing spatially multiplexed transmission paths with a plurality of antenna elements provided on a transmitter side and a plurality of antenna elements provided on a receiver side. This technique is indispensable not only to a wireless LAN but also to a system for mobile communication and a next-generation wireless communication system such as the IEEE802.16e (WiMAX).
In the MIMO communication system, transmitting data is distributed to a plurality of antenna elements of a transmitter, and respective distributed transmitting data are transmitted simultaneously at an identical frequency. Transmitted radio waves reach a plurality of receiving antenna elements via various propagation paths in a space. A receiver estimates a transmission function between the transmitting antenna and the receiving antenna, and executes arithmetic processing to reconstruct the original data. Generally speaking, in a case of a wireless apparatus that employs the MIMO communication system, a plurality of omnidirectional feeding elements, such as dipole antennas and sleeve antennas, are used. In this case, there has been such a problem that transmission quality is lowered because of an increased correlation among the feeding elements unless some contrivance is made so as to satisfactorily increase distances among the feeding elements or to provide polarized waves combinations different from each other by directing the respective feeding elements towards different directions.
As the prior art for solving this problem, it may be considered to use an array antenna apparatus such as a directivity adaptive antenna disclosed in Patent Document 1, for example. The array antenna apparatus of Patent Document 1 has such a configuration that three printed circuit boards are arranged so as to surround a periphery of a half-wave dipole antenna which is installed vertically on a dielectric support substrate. A high-frequency signal is supplied to the half-wave dipole antenna via a balanced feeding cable. In addition, each of the printed circuit boards has a back surface on which two pairs of parasitic elements provided in parallel, where one pair of the parasitic elements includes two printed antenna elements (each of which is a conductor pattern). In each pair of parasitic element, the two printed antenna elements are provided so as to be opposed to each other with a predetermined gap therebetween. Each of the printed antenna elements has an opposed-side end to which a through hole conductor is provided, and the through hole conductor is connected to an electrode terminal on a front side of the printed circuit board. In each of the parasitic elements, a varactor diode is mounted between two electrode terminals. Further, each of the electrode terminals is connected to a pair cable via a high-frequency stopping large resistor, and the pair cable is connected to applied bias voltage terminals DC+ and DC− of a controller that controls a directional pattern of the antenna apparatus. By switching over an applied bias voltage from the controller, reactance value of the varactor diode connected to the parasitic element changes. Therefore, electrical lengths of the parasitic elements are changed relative to the half-wave dipole antenna, and a planar directional pattern of the array antenna apparatus is changed.
It is possible to decrease the distances among the feeding elements by adopting an adaptively directional antenna such as the array antenna apparatus of the Patent Document 1 as an antenna for the MIMO communication, and by setting directivity of each of antennas so as not to cause a correlation among the antennas.